An optical lens is typically made of plastic or glass material and generally has two opposing surfaces which co-operate with one another to provide a required corrective prescription. When the positioning or shape of one of these surfaces with respect to the other is inaccurate, optical errors can occur.
Manufacturing of an optical lens to the required prescription requirements typically includes machining the surface of a semi-finished lens or lens blank. Typically, a semi-finished lens has a finished surface, for example the front surface and an unfinished surface, for example the back surface. By machining the back surface of the lens to remove material, the required shape and positioning of the back surface with respect to the front surface for the desired corrective prescription can be generated.
During manufacturing of the lens it is important that the semi finished lens is securely maintained in an accurate position on a lens holding unit during the various manufacturing operations in order to prevent the generation of optical errors.
Conventionally, a semi-finished lens is provided with engraved markings on the finished surface. The engraved markings define a reference system of the finished surface of the lens.
So as to manufacture accurately the unfinished surface of the semi-finished lens, the semi-finished lens is maintained by blocking the finished surface of the lens on a lens holding unit. Various materials may be employed to secure the semi-finished lens to the lens holding unit. These materials include for example glues or low temperature fusible metal alloys.
The use of such materials requires that the finished surface of the lens be protected prior to being blocked on the lens holding unit. A protective tape is generally placed on the finished surface prior to blocking the semi-finished lens.
The protective tape can make the viewing of the engraved markings, for example trough the semi-finished lens, difficult. Thus, it may be difficult to accurately determine the position in which the semi-finished lens is blocked. An inaccurate estimation of the position of the semi-finished lens on the lens holding unit may introduce an inaccurate position of the manufactured surface relative to the unfinished surface.